Known in the art are a number of pigments which are used in compositions of colour-changing heat indicators. For example a pigment comprising a double iodide of copper and mercury (copper tetraiodomercurate) has its critical temperature (i.e. temperature of colour change) of 65.+-.5.degree. C. and heat-resistance (ultimate temperature at which the ability of an indicator of reversing its colour is retained) of 90.degree.-100.degree. C. (cf. B. G. Abramovich "Heat Indicators and Changes Thereof", Moscow, Energija Publishing House, 1972; B. G. Abramovich, V. F. Kartavtsev "Colour Heat Indicators", Moscow, Energija Publishing House, 1978.
Known in the art is a pigment for a colour-changing indicator consisting of a double salt of mercury and silver iodides and a double salt of mercury and copper iodides (E. Chiric, Culori Termoscopie, "Metrologia Aplicata", 1957, No. 4). This pigment has critical temperatures within the range of from 45.degree. to 100.degree. C. However, its application is economically inefficient, since for its preparation use is made of expensive silver; furthermore, its preparation procedure necessitates safe techniques for handling a strong poisonous substance.
Also known is a colour-changing indicator containing, as a pigment, iodides of copper and mercury suspended in a rigid heat-resistant transparent synthetic matrix (cf. FRG Pat. No. 1,698,095). This colour-changing indicator has a critical temperature of 68.+-.5.degree. C. and a heat-resistance of from 127.degree. to 135.degree. C.
A pigment consisting of compounds of iodides of copper and mercury and ortho-oxyquinoline is incorporated in a composition of a reversible colour-changing heat-indicating coating (cf. USSR Inventor's Certificate No. 735620) with a critical temperature of 65.+-.5.degree. C. and heat-resistance of 140.degree.-150.degree. C.
These colour-changing heat indicators have a comparatively high critical temperature of colour change, 60.degree.-68.degree. C., thus considerably limiting their field of application, for example for visual detection of short-circuits in electrorefining of heavy non-ferrous metals, in particular copper. The degree of response of such heat-indicators to a short-circuit is low, especially during autumn-winter periods where the temperature of the controlled surface is below critical due to a considerable heat-transfer despite the short-circuit; the degree of response of the heat indicator to short circuits is only 26 to 37% of the total number thereof. Due to a specific character of the process of electrorefining of copper, the events of surpassing the heat-resistant temperature of the indicator should not be underestimated, wherefore the heat indicator becomes "burnt", i.e. loses its colour-reversibility.